CLINICAL RESEARCH: ECHOCARDIOGRAPHY
Broad-beam spectral Doppler sonification of the vena contracta using matrix-array technology
A new solution for semi-automated quantification of mitral regurgitant flow volume and orifice area
Thomas Buck, MD, FACC*,*,
Björn Plicht, MD*,
Peter Hunold, MD ,
Ronald A. Mucci, PhD ,
Raimund Erbel, MD, FACC* and
Robert A. Levine, MD, FACC
* West German Heart Center Essen, University Duisburg-Essen, Essen, Germany
Department for Diagnostic and Interventional Radiology, University Duisburg-Essen, Essen, Germany
Cardiac Ultrasound Laboratory, Massachusetts General Hospital, Boston, Massachusetts
Manuscript received August 9, 2004;
accepted October 6, 2004.
* Reprint requests and correspondence: Dr. Thomas Buck, West German Heart Center Essen, Department of Cardiology, University Duisburg-Essen, Hufelandstrasse 55, 45122 Essen, Germany (Email: thomas.buck{at}uni-essen.de).
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Abstract
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OBJECTIVES: The objective of this study was to evaluate broad-beam spectral Doppler sonification of the vena contracta using a matrix-array transducer for quantification of mitral regurgitation (MR).
BACKGROUND: Noninvasive assessment of the severity of valvular regurgitation remains challenging. A recent technique measures regurgitant flow directly at the vena contracta based on the product of velocity times backscattered Doppler power (proportional to orifice area). That approach, however, has been limited by relatively narrow conventional beamwidths. Matrix-array transducers, recently developed for three-dimensional imaging, can potentially provide broader beams. Therefore, we addressed the hypothesis that deliberate broadening of the Doppler beam can encompass larger regurgitant cross-sectional areas to capture a broader range of regurgitant flows.
METHODS: A matrix-array transducer system was modified to provide a three-dimensionally expanded spectral Doppler sample volume. Calculations of orifice area, flow rate, and regurgitant stroke volume (RSV) from Doppler power were automated on board a routinely used echocardiographic scanner and tested in vitro. In 24 patients with isolated MR, RSV was compared with magnetic resonance imaging (MRI) mitral inflow minus aortic outflow from phase-velocity maps.
RESULTS: The calculated flow rate and RSV correlated and agreed well with reference values in vitro (r = 0.98 to 0.99) and in patients (r = 0.93, mean difference 0.4 ± 3.2 ml, p = NS vs. 0), with sufficient sonification to measure flow orifices up to 0.85 cm in diameter. Agreement with MRI was comparable in 17 patients with central and seven with eccentric jets (p = NS vs. 0).
CONCLUSIONS: The broad-beam spectral Doppler technique provides accurate, largely automated quantification of regurgitant flow rate and integrated RSV directly at the lesion. The accuracy related to broader sonification is made possible by the new matrix-array transducer design.
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Abbreviations and Acronyms
| | CSA = cross-sectional area | | LV = left ventricle/ventricular | | MR = mitral regurgitation | | MRI = magnetic resonance imaging | | PVI = power-velocity integral | | ROA = regurgitant orifice area | | RSV = regurgitant stroke volume | | VC = vena contracta |
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Noninvasive assessment of the severity of valvular regurgitation remains an elusive clinical goal. It has received greater emphasis recently with the availability of valve repair techniques and their earlier use to preserve ventricular function and prevent clinical complications.
Existing Doppler methods are mainly indirect and limited in accuracy and applicability by simplifying assumptions and multiple computational steps (1–5). Imaging the vena contracta (VC) approaches the goal of direct lesion assessment (6,7), but varies with the two-dimensional view chosen and does not provide flow rate or regurgitant stroke volume (RSV). Routine applications of single-frame color Doppler techniques also have difficulty accounting for dynamic variations in orifice area and flow throughout systole (8–10).
Regurgitant flow can be measured directly at the lesion as the product of flow velocity and regurgitant orifice area (ROA). We recently showed that, because backscattered Doppler power reflects the number of scatterers (mainly red blood cells), the integral of power times velocity, or power-velocity integral (PVI) at the VC of a regurgitant jet is proportional to the volume flow rate (11). This provides both the instantaneous regurgitant flow rate and regurgitant volume integrated over systole. Using standard phased-array transducer design, however, that analysis has been limited by the relatively narrow size of conventional ultrasound beams that do not encompass larger flow cross-sectional areas (CSAs). Basically, this limitation of a narrow beam reflects the requirements of two-dimensional imaging for high resolution and a narrow beam; Doppler then provides a one-dimensional measure of velocity vectors along the beam (12,13). Therefore, we have proposed the hypothesis that deliberate broadening of the Doppler beam to create a spatially expanded disk-shaped sample volume can encompass larger regurgitant CSAs to capture the entire spectrum of regurgitant flow. Until recently, such broadening of a gated spectral Doppler sample volume had not been possible. Conventional phased-array transducers with a single row of ultrasound elements allow beam broadening only in the lateral dimension. Recently introduced matrix-array transducers, in contrast, permit beam broadening in both the lateral and elevational dimensions, within and perpendicular to a standard imaging sector, to produce a three-dimensionally expanded sample volume with axial, lateral, and elevational dimensions. The proposed quantitative approach, if verified, would be made possible by the advent of such matrix-array transducers.
To facilitate clinical application of this method, we further implemented a system entirely on board a routinely used clinical scanner. This system includes a special matrix-array transducer with electronic aperture control for generating a broad-spectral Doppler beam, as well as software for automated flow analysis that overcomes the need for complex off-line analysis and minimizes manual tracing. This system provides an instantaneous and automatically integrated flow rate and orifice area quantification and is suitable for routine clinical scanning. In this study, we assess the ability of this broad-beam spectral Doppler technique to quantify mitral regurgitant (MR) flow in vitro and in patients, as compared with magnetic resonance imaging (MRI) results.
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Methods
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Basic principle of PVI flow analysis.
Because backscattered power (P) is linearly proportional to the number of scatterers for a given hematocrit (14,15), backscattered power in the Doppler spectrum recoded from flow through a thin disk-like sample volume is linearly proportional to the sonified blood volume of moving scatterers and therefore will be linearly proportional to the CSA of flow as long as the area is encompassed by the beam (Fig. 1) (16,17):
where P(v) is the spectral Doppler power associated with a Doppler velocity; backscattered power from stagnant blood within the sample volume is eliminated by high-pass filtering and does not contribute to the power in the Doppler spectrum from rapidly moving blood (Fig. 1) (18).
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Figure 1 Schematic illustrating the proportionality of backscattered Doppler power and cross-sectional area (CSA) of flow: a broad flow (left) with twice the CSA of a narrow flow (right), and therefore twice the volume of moving scatterers encompassed by the sample volume, will return twice the Doppler power.
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Because the regurgitant flow rate equals CSA of flow times velocity (19), it is proportional to backscattered Doppler power times velocity, integrated over the VC velocity spectrum, or the PVI (18).
This linear proportionality holds only for laminar flow at the proximal VC of the jet, which is the narrowest portion of the jet directly below the valve orifice where flow recording avoids nonlinear increases in power caused by turbulent eddies and entrainment of fluid into the jet farther downstream (14,15,19,20).
In order to obtain clinically useful absolute flow values, power measurements from a broad measurement beam are calibrated in the same individual and at the same depth against power returning from a narrow reference beam of known CSA (CSAref) that lies entirely within the flow, using a coefficient that establishes the proportionality between power and flow CSA (11,17,18,21).
where Kcal is the calibration coefficient; Pref is the power measurement resulting from the narrow reference beam; Pmeas is the power measured by the broad measurement beam; and CSAflow is the unknown CSA of flow within the broad measurement beam. The calibration coefficient is applied to obtain instantaneous flow rate passing through the VC:
Total RSV is then obtained by integrating the instantaneous flow rate over the time period T of regurgitation:
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Broad-beam spectral Doppler technique.
To generate a three-dimensionally broadened spectral Doppler sample volume, we used a matrix-array probe (3.5 MHz; model 21215A, Philips Medical Systems, Andover, Massachusetts) with three parallel bars of 64 piezoelectric elements each in the elevational dimension (referred to as a one-and-a-half dimensional probe). This matrix-array transducer can generate both a narrow calibration beam and a broad spectral Doppler measurement beam using electronic beamwidth control over both the lateral and elevational apertures. Ultimately, the two beams need to be generated simultaneously to make calibration and measurement a one-step procedure. In previous studies using conventional phased-array transducers, the two beams were transmitted separately—one using a narrow aperture to generate a wide measurement beam and the other using a broad aperture to generate a narrow calibration beam (11). More efficiently, however, a single broad beam can be transmitted, and the measurement and calibration beams formed separately during the receive phase of the system using parallel processing circuits. Toward this goal, using the matrix array, the same narrow aperture was used to broadcast a broad beam, which was processed differently upon return to provide both measurement and calibration beams (albeit separately at this time, with the potential for simultaneous parallel processing in the near future) (Fig. 2).
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Figure 2 Principle of the broad-beam spectral Doppler technique. A matrix array with a reduced aperture (active transducer elements in darker gray) transmits a broad beam (top), which is processed differently upon return to provide both a broad measurement and a narrow calibration beam: the full matrix-array aperture is used to form a narrow receive beam during calibration with a narrow sample volume that lies entirely within the regurgitant flow (bottom left), whereas the reduced aperture forms the broad receive beam during measurement with a broad spectral Doppler sample volume encompassing the vena contracta (bottom right).
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In the current application, to produce the broadened spectral Doppler measurement beam, activating only the central transducer elements effectively reduced the transducer aperture to 37.5% in the lateral dimension and 50% in the elevational dimension. The same transmit beam and power were used for the calibration beam, but with a narrow beam generated on receive by using the full transducer aperture (Fig. 2). Having a single transmit beam and power minimizes the complexity of calculations for calibration. Sample volume axial length was set to 0.17 cm to maximize placement of the sample volume within the axial dimension of the VC throughout systole. Preliminary in vitro studies showed results to be relatively insensitive to variations in the axial placement of the sample volume of roughly 1.0 to 1.5 cm, consistent with an effective length of the VC.
Automated regurgitant flow analysis software.
When originally implemented, this technique required extensive off-line calculations on a separate computer (11). To increase suitability for clinical application, these processing and analysis steps were incorporated directly into software on board the ultrasound scanner itself that can access the original acoustic power values. In most systems, unmodified acoustic power values are not directly available, as only nonlinearly compressed and postprocessed signals are stored in image memory. To recover unmodified power values, we bypassed the postprocessing curves and inverted the nonlinear compression. An intuitive user interface was integrated into the touch-screen control panel of the ultrasound scanner (Fig. 3), with the following steps to obtain an absolute measure of MR stroke volume:
- 1 After the user locates the MR VC in the two-dimensional color Doppler sector scan, the system is switched to high pulse-repetition frequency Doppler, and the sample gate of the narrow calibration Doppler beam is placed in the VC flow, based on a combination of visualization and best-defined high-velocity VC Doppler profile.
- 2 The bounds of the narrow, high-velocity VC Doppler spectrum are defined using an algorithm that automatically finds and displays maximum and minimum velocity borders based on power thresholds (Fig. 3). The spectral Doppler data within these bounds are extracted from image memory for processing. Based on displayed outputs, the operator can revise the integration bounds manually.
- 3 Pressing "Analyze Cycle" initiates automated integration of power within the identified Doppler velocity spectrum and time period; this is stored for use in calibration.
- 4 Selecting "Calculate Coeff." directs the system to calculate the calibration coefficient as follows, based on the known CSA and measured power of the reference beam:
where CF is a programmed correction factor that corrects for the decrease in receive sensitivity of the measurement beam compared with the reference beam due to the smaller receive aperture for the measurement beam (see previous text); this value is established by the transducer design (CF = 23.73 at a depth of 10 cm). Simultaneously, the system automatically switches to produce the broad Doppler measuring beam and indicates to the user that the measurement stage has begun.
- 5 After the Doppler spectrum of the measurement beam has been delineated and the cardiac cycle of interest selected, when "Analyze Cycle" is pressed, the software computes instantaneous flow rates by automatically integrating power times velocity within the Doppler spectrum and applying the calibration factor. Finally, the flow rates are integrated over the selected time period to calculate and display total RSV along with mean flow CSA (Fig. 3).
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Figure 3 Automated regurgitant flow analysis software at work in the calibration stage (left) and measurement stage (right). (Top) Touch-screen panel with control keys. (Bottom) In vitro example of a narrow-spectrum Doppler signal from laminar flow at the vena contracta with the automatic border detection algorithm in operation. The measurement bars—S(ystole) and D(iastole)—specify a cardiac cycle of interest and the limits of integration over time. In the calibration stage (left), the software displays mean power and mean pixel intensity within the specified cardiac cycle. In the measurement stage (right), total flow volume (47.8 ml) and average flow cross-sectional area (average cross-sectional area 0.31 cm2) are displayed.
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In vitro studies.
Flow was driven from a Plexiglas chamber (5.7 cm diameter) simulating the left ventricle (LV) through an orifice 10 cm from the ultrasound transducer to a receiving chamber simulating the left atrium (11). Flow of a previously described blood-mimicking fluid (18) was measured by the broad-beam spectral Doppler method and directly compared with known steady flow rates of 10, 20, 30, 40, and 50 ml/s from a piston pump, as well as parabolic pulses of 10, 20, 30, 40, and 50 ml from syringe injections, all passing through circular orifices of 0.24, 0.38, 0.5, 0.57, 0.64, and 0.78 cm2 (diameters of 0.55 to 1.0 cm, corresponding to clinical lesions ranging from "mild to severe") (4,10).
Patient studies.
Initial clinical testing involved 24 routinely referred patients (age 55 ± 18 years; 13 males and 11 females) with isolated MR and a range of severity and etiologies, including seven eccentric and 17 central jets (Table 1), studied from a transthoracic apical approach with image and Doppler quality suitable for quantitative analysis. The RSV by the broad-beam spectral Doppler method was derived as the median of three independent measurements and compared with MRI obtained within 1 h of the Doppler study. This study was approved by the institutional Ethics Committee, and informed consent was obtained from all patients before examination.
MRI.
The RSV was obtained as mitral inflow minus aortic outflow with a 1.5-T system (Magnetom Sonata, Siemens Medical Systems, Erlangen, Germany). Phase-contrast cine acquisitions were obtained in planes aligned with the mitral annulus and orthogonal to the mid-ascending aorta. An ECG-triggered free-breathing through-plane phase-contrast sequence (repetition time 25 ms; echo time 4.8 ms; flip angle 15°; matrix 129 x 256; four averages) was used. By manually drawing regions of interest over the appropriate flow areas, phase-contrast velocity maps were acquired, integrated over time, and subtracted using built-in software (22,23).
Statistical analysis.
Automated broad-beam spectral Doppler flow measurements were compared with MRI reference values by linear regression. Agreement was assessed by plotting differences against reference values (or, in patients, the mean of calculated and reference values), comparing mean differences to zero by t test (24). Interobserver variability of broad-beam spectral Doppler measurements of RSV in patients was determined from two independent Doppler signal acquisitions and measurements performed by two observers (T.B. and B.P.).
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Results
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In vitro studies.
Narrow, high-velocity Doppler spectra characteristic of the VC were successfully recorded by the reference and measurement beams in all flow conditions, including spectra from parabolic pulses with maximal velocities up to 500 cm/s (mean 311 ± 110). Automated border definition of this velocity spectrum could be performed in all flow conditions, except the mildest flows through the smallest orifices (10 ml/s, 0.24 cm2), where weaker signals required manual tracing. Calculated steady flow rates correlated and agreed well with actual values for orifice diameters up to and including 0.85 cm (Fig. 4, top panel; r = 0.98, y = 0.97x + 0.9 ml/s, SEE = 2.4 ml/s) and over the entire velocity range, with a mean difference of –0.04 ± 2.4 ml/s (p = 0.90; NS vs. 0). Similar correlation and agreement were observed for pulsatile stroke volume (Fig. 4, middle panel; r = 0.99, y = 0.98x + 0.8 ml, SEE = 2.2 ml) for the same range of orifice diameters, with a mean difference of –0.3 ± 2.2 ml (p = 0.35; NS vs. 0). Flow rates and stroke volumes passing through circular orifices of 0.9 cm diameter or larger were underestimated because orifices of this size, which are clinically extreme (6,7,10), are currently incompletely assessed with available beam width and were not included in the regression (11,18). Correspondingly, there was good correlation between calculated flow CSA (effective orifice area) and actual (anatomic) orifice size for orifice diameters <0.9 cm (areas of 0.24 to 0.57 cm2), with underestimation only for larger diameters and areas in the clinically severe range (Fig. 4, bottom panel; r = 0.95, y = 0.74x + 0.01, SEE = 0.03 cm2, where the slope is <1, as expected when plotting effective versus anatomic orifice area, based on the coefficient of orifice contraction [25]).
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Figure 4 In vitro results for calculated versus actual flow rate in steady flow (top), calculated versus actual regurgitant stroke volume (RSV) in pulsatile flow (middle), and calculated average flow cross-sectional area (CSA) versus known anatomic orifice area and diameter (bottom). See text.
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Patient studies.
In all patients, high-velocity Doppler spectra of intensity suitable for quantification could be recorded throughout systole. Figure 5 shows examples in three patients with ischemic MR, degenerative mitral valve disease, and an eccentric as well as two central jets. In the patient studies, peak velocities by high pulse-repetition frequency and continuous wave Doppler agreed closely, with a mean difference of –3 ± 16 cm/s, p = 0.22 (NS vs. 0). Brachial systolic blood pressure values in the patients ranged from 90 to 175 mm Hg (mean 123 ± 23 mm Hg), which corresponds to peak orifice velocities of 4.3 to 6.3 m/s, assuming a left atrial pressure of 15 mm Hg (or slightly lower, as the brachial cuff pressure mildly overestimates LV pressure); the observed high pulse-repetition frequency peak velocities ranged from 4.0 to 6.3 m/s (mean 5.12 ± 0.58 m/s). Because of a weaker definition of the lower velocity border of the Doppler spectrum, using the current software, manual velocity border tracing was required in 15 patients. Calculated RSV correlated and agreed well with MRI values up to 40 ml (ROAs of 0.11 to 0.61 cm2) (Table 1), which extends into the clinically severe range (4,6,10) (Fig. 6; r = 0.93, y = 0.93 + 1.8 ml, SEE = 3.4 ml, mean difference 0.4 ± 3.2 ml, p = 0.57; NS vs. 0). There was mild underestimation only for one higher RSV, with a calculated effective ROA of 0.60 cm2 (consistent with the underestimation due to currently limited beam size, as shown in vitro). Results of flow CSA by broad-beam spectral Doppler (Table 1) were not compared with MRI because MRI did not provide direct CSA measurement. Agreement of RSV with MRI values was maintained over the entire velocity range, including the highest velocities. Results of RSV were comparable for patients with central or eccentric MR jets (mean difference 0.40 ± 3.1 ml vs. 0.33 ± 4.0 ml, NS by the t test). Interobserver measurements of RSV correlated well (r = 0.97, y = 1.03x – 1.1 ml, SEE ± 2.5 ml), with a small interobserver variability (mean difference –0.22 ± 2.4 ml, NS vs. 0), the variation coming only from selection and tracing of Doppler signals, because further flow calculations were automated.
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Figure 5 Examples of patient application of the broad-beam spectral Doppler method—maximal velocities. (Top) Patient with ischemic cardiomyopathy and a dilated left ventricle with poor global function and functional mitral regurgitation. Four-chamber view with color Doppler depicts a central regurgitant jet with a narrow proximal jet width in this view (broader in perpendicular views not shown). The power-velocity integral analysis of the manually traced narrow-spectrum vena contracta Doppler signal provided an average flow cross-sectional area of 0.30 cm2 and regurgitant stroke volume (RSV) of 28.7 ml, with a corresponding magnetic resonance imaging (MRI) RSV of 27.9 ml (moderate regurgitation). (Middle) Patient receiving long-term dialysis with
degenerated and calcified mitral leaflets but only mildly impaired left ventricular function. Color Doppler showed a large regurgitant jet into the dilated left atrium and reversed flow in the pulmonary vein. The PVI analysis provided a relatively large flow cross-sectional area (CSA) of 0.51 cm2, with an RSV of 35.4 ml (31.5 ml by MRI). Note the importance of the duration of regurgitation on RSV calculation, here causing a smaller RSV relative to the CSA compared with the patient above due to the shorter period of flow of 380 versus 470 ms; the velocities driving flow across the regurgitant orifice are also lower in this second patient, extending only to 4.5 m/s, as opposed to a peak orifice velocity of 5.0 m/s in the above patient (spectral tracings on the right). (Bottom) Patient with ischemic cardiomyopathy and leaflet malcoaptation, with an eccentric wall jet attached to the lateral left atrial wall. The PVI analysis of the vena contracta Doppler spectrum revealed a flow CSA of 0.33 cm2 and an RSV of 23.7 ml (30 ml by MRI). (In all three cases, maximal high pulse-repetition frequency Doppler velocities corresponded with left ventricular-to-left atrial pressure gradients estimated from systolic pressure (top: 508 cm/s vs. 120 mm Hg; middle: 466 cm/s vs. 100 mm Hg; bottom: 500 cm/s vs. 115 mm Hg, assuming a left atrial pressure of 15 mm Hg, or slightly lower, as brachial cuff pressure mildly overestimates left ventricular pressure.)
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Figure 6 Patient study results for calculated versus reference values of regurgitant stroke volume (RSV). BB Doppler = broad-beam spectral Doppler; MRI = magnetic resonance imaging.
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Discussion
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Quantification of MR stroke volume in routine clinical practice remains challenging in both accuracy and feasibility (1–5,26–28) The ideal approach would measure flow directly at the regurgitant lesion throughout the cardiac cycle for accurate integration of RSV. The integral of Doppler power times velocity of unidirectional, laminar flow at the VC of a regurgitant jet provides such a technique, but has until now been limited in its applicability by available beamwidth in transducers designed for high-resolution two-dimensional imaging (11,18). This study shows that recently developed matrix-array technology, for the first time, permits formation of cross-sectionally broadened spectral Doppler sample volumes that can overcome this limitation by encompassing flow CSAs up to the range of severe MR and can provide accurate RSVs both in vitro and in patients, as compared with independent standards. Compared with routine application where spectral Doppler is considered as a one-dimensional technique, practical application of the broad-beam spectral Doppler method, for the first time, necessitates all three dimensions of the Doppler sample volume, the two cross-sectional dimensions, and the sample volume height, to fully encompass a three-dimensional region through which laminar flow passes. The transducer and beams used in this study were able to capture regurgitant flow information accurately from orifice diameters of up to 0.85 cm and flow CSAs up to and including 0.6 cm2, extending the range of accuracy from mild-to-moderate with previous conventional transducers up to severe MR with the matrix array (4,6,10). Importantly, looking directly at the VC, accuracy was found to be persistent also for eccentric jets, which have been difficult to evaluate based on jet area (29), or using continuous-wave Doppler in previous studies (30). That is because the PVI method is relatively immune to variations in the Doppler beam-to-flow angle  : the cos decrease in measured velocity is canceled by a reciprocal increase in CSA relative to the beam, and any variations in attenuation with angle are dealt with by the dual-beam calibration. Further, full implementation of this entire process of beam formation and signal analysis directly on an available ultrasound scanner obviates the need for data transfer and off-line analysis, making the technique better suited for routine clinical practice.
Previous studies using standard one-dimensional phased-array transducer designs for Doppler power analysis were limited by a relatively narrow Doppler sample volume and therefore incomplete capture of regurgitant flow Doppler power and velocity (11,18,31–34). Experimental approaches to broaden sonification by annular array transducer design were limited to the non-imaging Doppler mode only and therefore difficult to apply in practice (17,21,35). Previous investigators aiming to measure flow through broader conduits (32,34) or fully opening heart valves (31,36,37) corrected the amplitude-weighted or power-weighted mean velocity from the sonified portion of the entire flow CSA based on a separately measured flow CSA (32,33) or simply determined a flow volume fraction (31,34,36,37); these flow estimates were inherently limited by incomplete flow sonification (31,33,34). The current approach addresses the more readily achievable goal of sonifying the regurgitant CSA, as opposed to the entire mitral or aortic annular areas.
Severity: Orifice area and RSV.
According to recently published American Society of Echocardiography recommendations (10), seven patients had severe MR based on an average effective regurgitant orifice area (EROA)  0.4 cm2, with 14 moderate and 3 mild cases. In addition, Grigioni et al. (38) and Enriquez-Sarano et al. (39) have shown that, in patients with LV dysfunction, criteria for MR severity should be reduced, commensurate with the diminished forward output, and even lower EROAs are associated with increased mortality and pulmonary hypertension. Regurgitant stroke volumes ranged up to 46 ml/beat and, in patients with LV dysfunction, were in Sarano's severe range (30 ml/beat by Doppler and/or MRI) in five patients. Potential reasons for lower RSV relative to EROA included: 1) a short regurgitant flow period; 2) low velocities due to low LV-to-left atrial pressure gradients; and 3) dynamic variations in EROA, as in ischemic MR, with a typical mid-systolic trough of EROA when driving pressure is highest. This points out a potential strength of the current approach compared with looking only at the EROA or VC itself, as it incorporates both orifice and velocity information instantaneously to provide an integrated assessment of flow rate and volume in addition to orifice area.
Study limitations and future perspectives.
Although the present broad-beam spectral Doppler technique provides broad sonification with a maximal beam CSA of 0.6 cm2, which can cover most clinical instances of MR, including severe lesions, ultimately this new matrix-array technology can be extended to encompass multiple and slit-like orifices along the mitral commissure. Recently developed and fully two-dimensional matrix-array probes with 50 x 60 = 3,000 active elements for three-dimensional volumetric imaging (40,41) can potentially enhance the ability to form and steer broad-beam Doppler sample volumes. Further beam broadening could be achieved even with current transducers by aperture narrowing, but this would critically sacrifice signal strength. Two-dimensional matrix-array probes could, in principle, overcome this obstacle by rapidly generating and steering broad or slit-like composite sample volumes expanded in three dimensions, with preserved signal strength and a uniform acoustic power profile for measuring regurgitant flow. It must be recognized that this approach applies only for laminar, unidirectional flow, such as at the VC. The Doppler high-pass filtering effectively eliminates backscattered signals from low-velocity targets such as tissue, whereas power from scatterers outside the laminar flow CSA does not contribute to the Doppler spectrum of rapidly moving blood within the VC. From this, it becomes clear that this approach cannot be extended to quantify the multidirectional and three-dimensional flows present within most of the ventricles and atria (42).
In addition, the approach in the current study sets the foundation for making the measurement and calibration steps simultaneous in order to maximize speed and accuracy. The current application generates the same beam pattern for both measurement and calibration (Fig. 2), but separately collects returning signal with different receive apertures in order to record the broad and narrow beams. Phased-array parallel processing can achieve this same dual analysis from a single transmit beam by processing returning sound in parallel from narrower and broader sets of transducer elements. This processing capability already exists on current scanners to increase the frame rate and can readily be adapted to make regurgitant flow quantification into a one-step acquisition process.
Conclusions and implications.
We have demonstrated a new Doppler method for accurate quantification of RSV by integrating backscattered Doppler power times velocity from a broadened spectral Doppler sample volume that fully encompasses regurgitant flow at the VC. This approach overcomes the limitations of existing methods by measuring flow directly at the regurgitant lesion. This method is made possible by the advent of matrix-array technology. It is fully implemented on board a routinely used ultrasound scanner, and most individual steps of Doppler spectral analysis and calculation are automated to promote rapid and routine application and improve our evaluation of the severity of regurgitation.
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Footnotes
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Dr. Buck was supported by grants Bu1097/2-1 and Bu1097/2-2 from the Deutsche Forschungsgemeinschaft, Bonn, Germany. The work was supported in part by National Institutes of Health (Bethesda, Maryland) grants R01 HL38176, HL53702, and K24 HL67434.
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